This is old information and out of date.
Audi's current head of R&D, Dr Ulrich Hackenberg has clearly stated in an interview with this months EVO magazine the following:
- Audi will stick with large V-configuration petrol engines because they're premium.
- The new petrol V's will be turbocharged, a single turbo for the regular range and twin-turbos for the performance variants.
- The supercharged V6 in the S4 is history - the next S4 will have an all-new twin-turbo V6
- The next RS4 will lose its V8 for a highly tuned version of the new twin-turbo v6 possibly with electric turbos.
- The twin-turbo 3-litre TDi V6 engine fits in the new A4, but the business case is not currently viable for an 'S4D'
He mention a load more other stuff but these are the items pertaining to the S4 and RS4
I have tried to speak to my friends in the Audi powertrain department and even though they are not saying much they did confirm the the EVO interview is pretty accurate.
That said, I am very familiar with the automotive design process having spent many years in it and things change and multiple streams of design are worked on at any one time. This means a change of direction can be made at any point. Remember the 1st S5's, they got the old S4 V8 (340bhp) because the V6T wasn't ready when the chassis was designed, it later got changed for the supercharged unit.
You may see the 1st B9 S4 with the V6T engine until the twin-turbo is absolutely spot on then they'll swap it out,